The present invention relates generally to the art of electrical circuit interrupting devices. More particularly, the invention relates to a method for rapidly disconnecting a current path between two electrical conductors in response to an overcurrent condition or other circuit malfunction, such as a short circuit.
A large number of devices and methods are known for interrupting electrical power between conductors in response to overcurrent conditions, such as short circuits, phase loss, ground faults and the like. Such devices are typically designed into both residential and industrial electrical system for protecting electrical wiring, as well as devices such as contactors, motor starters, appliances and electric motors. In general, such protective devices include fuses and circuit breakers. Fuses are typically sacrificed by the overcurrent condition and must be thereafter replaced. Circuit breakers, on the other hand, typically physically open contacts in response to a tripping event and may thereafter be reset, either automatically following a cooling period, or by physical intervention of a user.
While existing circuit interrupting approaches of this type offer a range of response times and protection characteristics, they are not without drawbacks. For example, in certain environments and applications where extremely rapid power interruption is required, semiconductor fuses generally offer satisfactory response time, on the order of 0.6 milliseconds. However, such fuses are relatively expensive and must be physically replaced following a tripping event. While circuit breakers of known design may be reset, thereby avoiding the additional cost of replacement after a tripping event, they are typically substantially slower than fuses, having turnoff times (i.e. time to open and interrupt power) of typically 4 milliseconds. Moreover, the let-through energy in such devices increases as a function of the cube of their turnoff time, so long as the current rise is controlled by the source voltage and the circuit inductance, which is typically the case for a hard fault. Thus, circuit breakers responding in twice the time as fuses let through some eight times the energy, increasing the risk of damage to wiring or electrical devices intended to be protected.
In the case of circuit breakers, rapid turnoff of current is typically limited to the rate at which arcs generated during opening of the circuit interrupter at the heart of the device can be extinguished. While such devices can rely on a natural zero crossing in alternating current sources, such techniques are generally much too slow to adequately protect sensitive loads. Instead, to minimize the turnoff time, circuit breakers must develop a voltage opposing the arcs in a very short time. Several sources of opposing voltage in circuit breakers include steady state ohmic resistance of the arcs, space charge voltages at the points of arc attachment, induction voltages due to motion of the arcs (or conductors) through a magnetic field, and transient arc voltages resulting from the initial formation and growth of the arcs. Commuting arcs into a series of splitter plates is an established method of increasing the number of space charges opposing a fault current in conventional circuit breakers. However, such techniques do not provide a sufficiently rapid turnoff time to protect sensitive loads. Moreover, while circuit breakers have been designed to use induction effects, these have typically been physically large and expensive. Finally, steady state ohmic resistance of arcs is typically too small to permit reduction of fault current necessary for circuit breaking action.
There is a need, therefore, for an improved method for interrupting electrical power between two conductors that offers extremely short turnoff times, while not requiring replacement of expendable elements such as fuses. Moreover, there is a need for an improved method that is susceptible to implementation in relatively simple devices such as circuit breakers. Such devices should be capable of responding both to very rapidly occurring overcurrent conditions, such as short circuits, and to more gradually changing conditions associated with other types of overloads.